The area surrounding the Río Aros canyon and upper Río Yaqui valley constitute one of northwestern Mexico's largest and least fragmented wildlands, and is near the boundaries of subtropical and temperate biogeographic regions. Because of its remote and wild character, biogeographic setting, and increasing interest in the region by conservationists and naturalists, we surveyed the river corridor and major tributaries to document biological diversity in the region. In July and August of 2005, we traversed 184.8 km, of the Ríos Aros and Yaqui in inflatable boats from Nátora to El Río, conducting a preliminary biological inventory of plants, aquatic invertebrates, amphibians and reptiles, birds, and mammals along the river corridor and in tributary canyons. We report range extensions for species of plants (Ficus pertusa, Sideroxylon persimile, Tabebuia impetiginosa), aquatic invertebrates (Buenoa albida, B. thomasi, Macrovatellus mexicana, Martarega mexicana,), herpetofauna (Drymarchon corais, Terrepene nelsoni) and observations of birds at the edge of their ranges (Ara militaris, Haliaeetus lecuocephalus, Icterus wagler, Vireo flavoviridis). In total we observed 102 species of aquatic invertebrates, 8 amphibians, 13 reptiles, 80 birds, and 18 mammals during an initial assessment of the biological diversity of these taxa in the region. We hope that this preliminary assessment will aid in future conservation and management efforts in the Río Aros region.